ATOMIC ABSORPTION AND ATOMIC ATOMIC ABSORPTION AND ATOMIC FLUORESCENCE SPECTROMETRYFLUORESCENCE SPECTROMETRY

Chap 9Chap 9

Source Modulation

Interferences in Atomic AbsorptionInterferences in Atomic Absorption

SpectralSpectral

ChemicalChemical

Atomic Fluorescence SpectroscopyAtomic Fluorescence Spectroscopy

AA Source ModulationAA Source Modulation

• Need to eliminate emission from analyte atomsNeed to eliminate emission from analyte atoms• Source beam is choppedSource beam is chopped

Chopperplacedhere

Beam chopper forBeam chopper for subtracting flame subtracting flame background emissionbackground emission

(a)(a) Lamp and flameLamp and flame

emission reach detectoremission reach detector

(b)(b) Only flameOnly flame

emission reachesemission reaches

detectordetector

(c) Resulting signal(c) Resulting signal

ATOMIC ABSORPTION AND ATOMIC ATOMIC ABSORPTION AND ATOMIC FLUORESCENCE SPECTROMETRYFLUORESCENCE SPECTROMETRY

Chap 9Chap 9

Source Modulation Source Modulation

Interferences in Atomic Absorption

Spectral

Chemical

Atomic Fluorescence SpectroscopyAtomic Fluorescence Spectroscopy

Spectral InterferencesSpectral Interferences

1) Undesired signals overlapping analyte signal1) Undesired signals overlapping analyte signal

e.g., V at 308.11 nm near Al at 308.215 nme.g., V at 308.11 nm near Al at 308.215 nm

∴∴ use Al at 309.27 nmuse Al at 309.27 nm

2) combustion products 2) combustion products broadband absorption broadband absorption

• subtract signal from a blanksubtract signal from a blank

3) matrix interferences (3) matrix interferences (most seriousmost serious))

• use higher Tuse higher T

• use radiation buffer use radiation buffer

• add excess of known interferentadd excess of known interferent

Other methods of correcting for matrix effectsOther methods of correcting for matrix effects

• Continuum Source (DContinuum Source (D22) Correction Method) Correction Method

• DD22 lamp provides continuum lamp provides continuum

• UV abs. is subtracted from analyte signalUV abs. is subtracted from analyte signal

• Fig. 9-14Fig. 9-14

Fig 9-14Fig 9-14 Continuum-sourceContinuum-source background correction

Other methods of correcting for matrix effectsOther methods of correcting for matrix effects

• Continuum Source (DContinuum Source (D22) Correction Method) Correction Method

• DD22 lamp provides continuum lamp provides continuum

• UV abs. is subtracted from analyte signalUV abs. is subtracted from analyte signal

• Fig. 9-14Fig. 9-14

• Zeeman Background CorrectionZeeman Background Correction

• based on splitting of absorption lines based on splitting of absorption lines by a magnetic fieldby a magnetic field

• lines absorb different polarizationslines absorb different polarizations

Fig 9-14 Background correction by Zeeman effectFig 9-14 Background correction by Zeeman effect

• B field splits atomic absorption lines (Zeeman effect)

• Line absorption differs with polarization of source

Chemical interferencesChemical interferences (very common) (very common)

1) Certain components of the sample decrease 1) Certain components of the sample decrease the extent of atomizationthe extent of atomization

e.g., SOe.g., SO442-2- and PO and PO44

3-3- hinder atomization of Ca hinder atomization of Ca2+2+

• Add Add releasing agentreleasing agent: Sr: Sr++, La, La3+,3+, etc. etc.

• Add Add protecting agentprotecting agent: EDTA, hydroxyquinoline: EDTA, hydroxyquinoline

2)2) Ionization interferenceIonization interference

•Occurs when OOccurs when O22 or N or N22O is oxidantO is oxidant

•Analyte ionizes and releases electrons at high TAnalyte ionizes and releases electrons at high T

M M ⇌⇌ M M++ + e + e--

Table 9-2 Degree of Ionization with temperatureTable 9-2 Degree of Ionization with temperature

•Add Add ionization supressorionization supressor: K, Rb, Cs salts: K, Rb, Cs salts

•Provides high concentration of electrons to flameProvides high concentration of electrons to flame

•M M ⇌⇌ M M++ + e + e- - shifted to the leftshifted to the left

Fig 9-17 Effect of K as ionization suppressor for CaFig 9-17 Effect of K as ionization suppressor for Ca

Table 9-3 Detection limits (ppb)Table 9-3 Detection limits (ppb)

for selected elementsfor selected elements

ATOMIC ABSORPTION AND ATOMIC ATOMIC ABSORPTION AND ATOMIC FLUORESCENCE SPECTROMETRYFLUORESCENCE SPECTROMETRY

Chap 9Chap 9

Source ModulationSource Modulation

Interferences in Atomic AbsorptionInterferences in Atomic Absorption

SpectralSpectral

ChemicalChemical

Atomic Fluorescence Spectroscopy

Absorption and fluorescence by atoms in a flameAbsorption and fluorescence by atoms in a flame

Set-up for a Luminescence ExperimentSet-up for a Luminescence Experiment

• If source is at fixed λ and monochromator is scanned an emission spectrum results.

• If source is scanned, and monochromator is at a fixed λ, an excitation spectrum results.

Fig. 7-1 (b)

90°